A great Pleco at only 6" is the L264 Leporacanthicus Joselimai. Mine has survived a tank with 5 large silver aros and a 15" Reticulated Pimelodid. Also the Queen Arabesque L260 at about 5". Or the "Royal" plecos are a good choice. I prefer the L330 Spotted Royal Pleco or Full Spotted Pleco at about 8 - 10". Also a fairly small but beautiful pleco is the Green Phantom L200 (See the Blue Phantom, looks very similar) about 8". All are medium small plecos but all very pretty additions to the tank.